The distance of a cloud from the Earth has a big influence on its appearance. Air temperatures decrease with altitude, so clouds that form closer to earth are made mostly of water droplets, while higher clouds tend to be composed of ice crystals. Mid-level clouds often contain a mix of both. This difference in composition influences how clouds look: water-based clouds, like cumulus, have crisp edges and a solid appearance, while icy clouds, such as cirrus, are usually more transparent and diffuse. Air movement also affects cloud shapes. As warm, moist air rises—a process known as convection—it cools and condenses, forming clouds. But something interesting happens in the process: as water vapor condenses, it releases heat, which warms the surrounding air. The warmer air becomes less dense than the surrounding cooler air, making it more buoyant. This increased buoyancy causes the air parcel to rise even faster.